Absorptiometric and fluorimetric study of solvent dependence and prototropism of benzimidazole homologues

Abstract

Electronic absorption and fluorescence spectra of benzimidazole and six of its derivatives in different solvents and at various pH values have been studied. The dual fluorescence observed in dilute acid solutions and the similarity of the fluorescence excitation spectra to the absorption spectra indicate the existence of several excited states of benzimidazoles. The emitting transition of neutral benzimidazoles is πâ†�π*; in the case of the monocations the short-wavelength fluorescence band is from the πâ†�π* transition and the long-wavelength band arises from charge transfer. The pK_a values for various prototropic reactions in aqueous solutions in the S₀ and S₁ states have been calculated, and the proton-induced fluorescence quenching of the monocations is discussed.